1. Field of the Invention
This invention relates to a sequential chirp modulation-type spread spectrum communication system hereafter abridged as (SS-SC system), especially to the sequential chirp modulation-type spread spectrum communication system relating to FH-type which realizes fine transmission quality at low cost.
2. Description of the Related Art
A spread spectrum communication system (SS system) is known as a communication system having transmission characteristics resistant to variation. The spread spectrum communication system is a system which transmits a narrow band information signal to a transmitting channel after spreading the spectrum at a wide band by modulating with the PN (pseudo noise code) sequence having high clock frequency, detects correlatively the receiving signal at the receiving side, and obtains high S/N after demodulating by de-spreading the spectrum.
Since this spread spectrum communication system has some excellent advantages, e.g., its resistance to narrow band noise and the variation of transmission line characteristics (e.g., fading) and its higher secrecy, its applications to various fields has been investigated and developed.
The different kinds of SS communication systems are the DS system (direct sequence system), FH system (frequency hopping system), TH system (time hopping system), chirp system (pulse FM system) and hybrid system, but most of them are based on the DS system and FH system.
The DS system performs wide band transmission. Prior to the transmission, in the DS system carrier was first modulated by transmission signal, and then modulated by Pseudo-Random binary code sequences which has sufficient length and rapid bid rate, and demodulated the local signal by modulating again by BPSK, etc., using the same codes at the receiving side.
On the other hand, the FH system is a system which prepares a sufficient number of carrier wave frequency slots in the specified frequency area, changes the carrier wave frequency dispersedly (discontinuously) according to a predetermined schedule and demodulates this signal. This type has less problem about "near far problem" but the hard-ware cost is relatively high if higher hopping speed is used, because it is not possible to use single receiver using PLL synthsizer (lack of response time to frequency change requirement). Thus, it is necessary to use a multiple receiver that corresponds to frequency slots used in the system and the system reliability becomes lower because of the system complexity.
The processing gain (PG) for this system is defined simply by the number of frequency slot number in which the carrier hops. Therefore in the high speed hopping system, PG also pushes up the system hard-ware cost.
These two basic methods are often used in the mixed condition. It is called "hybrid type". The mixing enables further expansion of the communication band-width.
As the chirp system relates to this invention to some extent, it will be explained briefly as follows. The chirp system is a system which generates a signal whose frequency is gradually changed by a pulse sine wave during the pulse lasting period by SAW filter, and transmits and receives the signal. However, this system is only utilized for a radar system and it has not been used for general transmission.